LCD with first and second circuit regions each with separately optimized transistor properties

ABSTRACT

A large number of pixels PXL are arranged in a matrix fashion in a display region DSP on an insulating substrate. Disposed around the display region DSP are a drain-side pixel-driving circuit including a drain shift register DSR, a digital-to-analog converter circuit DAC, a drain level shifter DLS, a buffer BF and sampling switches SSW; and a gate-side pixel-driving circuit including a gate shift register GSR and a gate level shifter GLS, and various kinds of circuits. Current mobility of thin film transistors constituting a circuit region SX requiring high-speed operation of these pixel-driving circuits is improved by optimizing a combination of plural layouts, arrangements and configurations for the respective circuits to meet the specifications special for the respective circuits.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of nonprovisional U.S. applicationSer. No. 10/772,431 filed on Feb. 6, 2004. Priority is claimed based onU.S. application Ser. No. 10/772,431 filed on Feb. 6, 2004, which claimsthe priority of Japanese Application 2003-176281 filed on Jun. 20, 2003,all of which is incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image display device, andparticularly to an image display device provided with a circuitemploying a thin film transistor capable of high-speed operation.

2. Description of Prior Art

Polycrystalline silicon thin film transistors (hereinafter also calledpolycrystalline silicon TFTs) have been developed as active elementsforming pixels or pixel-driving circuits for active-matrix type liquidcrystal display device (liquid crystal displays), organic light-emittingdisplay devices (organic EL displays), and image sensors.Polycrystalline silicon TFTs have an advantage of greater drivingcapability over other driving circuit elements and peripheral circuitsformed by polycrystalline silicon TFTs can be mounted on the samesubstrate mounting the pixels thereon.

In a case in which polycrystalline silicon TFTs are used for large-sizedliquid crystal display devices such as TV receivers and large-sizedmonitors, the polycrystalline silicon TFTs are fabricated on a glasssubstrate which is an insulating substrate forming an active substrateof the display device (or the so-called active-matrix substrate),because of cost limitations. In a process of fabricating TFTs on a glasssubstrate, processing temperatures are determined by a withstandtemperature of the glass substrate.

For formation of high-quality polycrystalline silicon films (hereinafteralso called polycrystalline silicon films) on a glass substrate(hereinafter also called simply a substrate), crystallization by excimerlaser is utilized as disclosed in Boyce, J. B. and P. Mei: “3. LaserCrystallization for Polycrystalline Silicon Device Application,”Technology and Applications of Amorphous Silicon (2000), pp.94-146,Springer 2000.

In a case where integrated circuits of higher performance are to beincorporated into a pixel-driving circuit, polycrystalline silicon TFTsof higher performance need to be realized. Crystallization by usingsolid-state laser provides polycrystalline silicon films formed ofcrystalline grains having their sizes extended in a scanning directionof laser and uniform crystalline grain widths in a directionperpendicular to the scanning direction of laser, and having an evensurface, as described in Hara, A. et al.: “High Performance Poly-Si TFTson a Glass by a Stable Scanning CW Laser Lateral Crystallization,” pp.747-750, conference papers, International Electron Devices meeting(Washington D.C., 2001), and Hatano, M. et al.: “12.4 L: Late-NewsPaper: Selectively Enlarging Laser Crystallization Technology for Highand Uniform Performance Poly-Si TFTs,” pp. 158-161, SID 02 DIGEST,Society for Information Display, International Symposium Digest 2002,for example. Polycrystalline silicon TFTs formed of the thus obtainedpolycrystalline silicon films were reported to have improvedperformances of thin film transistors.

SUMMARY OF THE INVENTION

Conventional polycrystalline silicon films crystallized by using excimerlaser and used for thin film transistors have small crystalline grainsizes, and little anisotropy in shape of crystalline grains. Therefore,regardless of which direction thin film transistors were fabricated toorient in on a substrate, their performances remained much the same.Because of this, the arrangement of thin film transistors on thesubstrate has been determined with the principal object of minimizingthe area occupied by the thin film transistors, and as a necessaryconsequence, there have been various thin film transistors which areoriented in various directions on the same substrate.

As explained above, crystallization by using solid-state laser providespolycrystalline silicon films formed of crystalline grains having theirsizes extended in a scanning direction of laser and uniform crystallinegrain widths in a direction perpendicular to the scanning direction oflaser, and having an even surface. Performances of TFTs fabricated fromtheses polycrystalline silicon films makes it possible to incorporatecircuits which could not be mounted on an active-matrix substrate, andthereby to make the active-matrix substrate highly functional. However,since silicon crystals were highly anisotropic in the conventionalpolycrystalline silicon films, conventional layouts could not obtainperformances required for circuit operations in some cases.

It is an object of the present invention to provide an image displaydevice provided on its insulating substrate with (1) a pixel section(also called a pixel region or a display region) having a large numberof pixels arranged in a matrix fashion, and (2) a pixel-driving circuitsection for driving the pixel section and formed of high-performancethin film transistor circuits operating on high current mobility(electron mobility, hole mobility).

The present invention is not limited to the conversion ofpolycrystalline silicon film such that semiconductor films formed on aninsulating substrate of an image display device is improved to exhibitcharacteristics capable of providing the above-mentioned high currentmobility, but is equally applicable to the conversion of semiconductorfilms formed on other substrates, for example, a silicon wafer.

To achieve the above-mentioned object, the present invention is carriedout as follows:

Initially, a polycrystalline silicon film is obtained by converting anamorphous silicon film deposited over an entire area of an insulatingsubstrate into a polycrystalline silicon film by irradiating excimerlaser light over an entire surface of the amorphous silicon film, or aninsulating substrate having a polycrystalline silicon film thereon isprepared by depositing a polycrystalline silicon thereon by using achemical vapor deposition method (a CVD method).

Next, pulse-modulated laser light or quasi-CW laser light fromsolid-state laser is selectively irradiated onto a portion of thepolycrystalline silicon film in a driving circuit region disposed aroundthe pixel region on the above-explained insulating substrate while thelaser is scanned in a specified direction to obtain discontinuousconverted regions comprised of even band-shaped-crystal polycrystallinesilicon films formed of crystalline grains having their sizes extendedin the scanning direction of the laser light and uniform crystallinegrain widths in a direction perpendicular to the scanning direction ofthe laser light.

The above-mentioned discontinuous converted regions are selected to berectangular, and desired circuits are fabricated in the rectangulardiscontinuous converted regions, respectively. At this time, each ofTFTs forming the circuits is oriented in a direction optimized to meet aspecification of corresponding one of the circuits by considering TFTcharacteristics depending upon an orientation of a channel of the TFTwith respect to the scanning direction of the laser light, in order toaccomplish the above-mentioned object of the present invention.

In this specification, the above-explained method of fabricatingdiscontinuous converted regions comprised ofgenerally-band-shaped-crystal silicon films by using irradiation ofpulse-modulated laser light or quasi-CW laser light will be called SELAX(Selectively Enlarging Laser Crystallization).

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, in which like reference numerals designatesimilar components throughout the figures, and in which:

FIG. 1 is a plan view schematically illustrating an outline of an imagesection, a pixel-driving circuit section and other necessary circuitsfabricated on a glass substrate which serves as an active-matrixsubstrate constituting an image display device.

FIG. 2 is a table summarizing specifications required for respectivecircuits fabricated in respective driving circuit regions of theactive-matrix substrate, and advantages obtained from thespecifications.

FIGS. 3A(1) to 3B(2) are schematic diagrams illustrating a manner inwhich an amorphous film is converted into a polycrystalline silicon filmhaving good quality.

FIGS. 4A and 4B are plan views schematically illustrating relationshipsamong a layout of a thin film transistor, scanning directions ofsolid-state laser light and grain boundaries.

FIG. 5 is a graph showing a comparison in transfer characteristicbetween two different layouts of transistors fabricated within virtualtiles VTL.

FIG. 6 is a plan view of a principal portion for illustrating an exampleof a layout of a pixel on an insulating substrate in an example of thepresent invention.

FIG. 7 is a plan view of a principal portion for schematicallyillustrating an example of a layout of one stage constituting adrain-side shift register DSR on the substrate.

FIG. 8 is a circuit diagram of a logic circuit corresponding to onestage constituting the drain-side shift register DSR.

FIG. 9 is a timing chart for explaining operation of the shift registerDSR shown in FIG. 7.

FIG. 10 is a graph showing a difference in characteristics due to adifference in film thickness between silicon oxide films used as gateinsulating films by plotting drain currents against gate voltages.

FIGS. 11A and 11B are schematic cross-sectional views of thin filmtransistor structures in one case in which impurity-injected layers oflow concentration are present in boundaries between a channel region anda source region and between the channel region and a drain region, andin another case in which a gate is disposed over impurity-injectedlayers of low concentration, respectively.

FIG. 12 is a plan view schematically illustrating a concept of anotherconfiguration example of an active-matrix substrate in accordance withthe present invention.

FIGS. 13A(1) to 13B(2) are illustrations for explaining a method ofpositioning regions to be irradiated by solid-state laser light fordetermining positions of virtual tiles VTL.

FIG. 14 is an exploded perspective view for schematically explaining aconfiguration example in which an image display device of the presentinvention is applied to a liquid crystal display device.

FIG. 15 is an exploded perspective view illustrating a configurationexample in which an image display device of the present invention isapplied to an organic EL display device.

FIG. 16 is a plan view of the organic EL display device obtained byassembling the constituent elements shown in FIG. 15.

FIG. 17 is an external view of an example in which an image displaydevice of the present invention is incorporated into a display sectionof a personal computer or a TV receiver.

FIG. 18 is an external view of an example in which an image displaydevice of the present invention is incorporated into a display sectionof a mobile phone.

FIG. 19 is an external view of an example in which an image displaydevice of the present invention is incorporated into a display sectionof a portable digital terminal.

FIG. 20 is an external view of an example in which an image displaydevice of the present invention is incorporated into a display sectionof a video camera.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, the embodiments in accordance with the presentinvention will be explained in detail by reference to the drawings. Herea case will be explained in which the present invention is applied to aliquid crystal display device which uses a glass substrate as itsinsulating substrate.

FIG. 1 is a plan view schematically illustrating an outline of an imagesection, a pixel-driving circuit section and other necessary circuitsfabricated on a glass substrate which serves as an active-matrixsubstrate constituting an image display device. In the case of a liquidcrystal display device, a liquid crystal material is filled in a spacebetween the active-matrix substrate and a counter substrate providedwith color filters or the like thereon and attached to the active-matrixsubstrate by an adhesive. The counter substrate provided with colorfilters are called the color filter substrate.

Here consider a case in which the active-matrix substrate is adapted forthe image display device of the line at-a-time scanning type. A majorportion of the glass substrate SUB is occupied by a pixel region DSP. Aplurality of pixels PXL are arranged in a matrix fashion in the pixelregion DSP, and each of the pixels PXL is disposed at an intersection ofone of data lines DL and one of gate lines GL. Each of the pixels PXL iscomprised of a TFT serving as a switch and a pixel electrode. In thisexample, the switch is illustrated as a double gate comprised of twoTFTs, but the switch may be comprised of a single gate or a multiplegate.

Disposed outside the pixel region DSP on the glass substrate SUB aredriving circuit regions formed with circuits for supplying drivingsignals to a large number of pixels PXL fabricated within the pixelregion DSP.

Disposed along one of the long sides of the pixel region DSP (at the topside in FIG. 1) are a shift register DSR which functions for displaydata in digital form to be read into a digital-to-analog converter DACsuccessively, the digital-to-analog converter DAC for outputting thedigital display data as gray scale signals, a level shifter DLS forproducing desired gray scale voltages by amplifying the gray scalesignals from the digital-to-analog converter circuit DAC, buffers BF,and sampling switches SSW which function to apply gray scale voltages ofopposite polarities to adjacent ones of the pixels PXL.

On the other hand, disposed along one of the short sides of the pixelregion DSP (at the left-hand side in FIG. 1) are a shift register GSRfor opening the gates of the pixels PXL successively and a level shifterGLS. Disposed in the vicinities of the above-mentioned circuits are aninterface IF for performing the signal conversion of image datatransferred from a system LSI circuit SLSI, a gray scale signalgenerator circuit SIG, clock signal generator circuits CLG forgenerating clock signals used for timing control of the respectivecircuits, and the like.

Among the above-mentioned circuits, since the interface IF, the clocksignal generator circuits CLG, the drain-side shift register DSR, thegate-side shift register GSR and the digital-to-analog converter circuitDAC handle digital signals, they are required to perform high-speedoperation. Further, they are required to be driven at low voltages forreduction of power consumption.

On the other hand, the pixel PXL is a circuit for modulating lighttransmission through a liquid crystal layer by applying a voltage to theliquid crystal layer, and therefore they need to driven at high voltagesfor producing gray scale levels. Moreover, for retaining a signalvoltage for a specified period of time, thin film transistors whichperform switching operation need to have small leakage currents. Sincethe drain-side level shifter DLS, the gate-side level shifter GLS, thebuffers BF and the sampling switches SSW are coupled between thelow-voltage driving circuits and the high-voltage driving circuits, andthey supply high-voltage analog signals to the pixels PXL, they arerequired to perform high-voltage driving operation. As explained above,in the case in which circuits for displaying of images are fabricated onthe glass substrate SUB, plural TFTs having specifications conflictingwith each other need to be fabricated on the same substrate.

FIG. 2 is a table summarizing specifications required for respectivecircuits fabricated in the respective driving circuit regions of theactive-matrix substrate, and advantages obtained from thespecifications. Since the specifications required for the circuitsfabricated in the respective regions and the advantages obtainedtherefrom are described in the table of FIG. 2, their explanations areomitted here.

To meet the required specifications summarized in FIG. 2, in thisexample, by scanning and selectively irradiating pulse-modulated laserlight on portions intended for the interface IF, the clock signalgenerator circuits CLG, the drain-side shift register DSR, the gate-sideshift register GSR and the digital-to-analog converter circuit DAC shownin FIG. 1, discontinuous converted regions comprised ofband-shaped-crystal silicon films are obtained which are converted suchthat crystalline grains have their grain boundaries continuous in thescanning direction of the laser light. These discontinuous convertedregions are denoted by reference symbols SX in FIG. 1.

FIGS. 3A(1) to 3B (2) are schematic diagrams illustrating the manner inwhich an amorphous film is converted into a polycrystalline silicon filmhaving good quality. FIG. 3A(1) is a perspective view of a substrate,and FIG. 3A(2) is a plan view of the substrate of FIG. 3A(1), as viewedfrom above the substrate of FIG. 3A(1). FIG. 3B(1) is a perspective viewof a substrate, and FIG. 3B(2) is a plan view of the substrate of FIG.3B(1), as viewed from above the substrate of FIG. 3B(1).

As shown in FIGS. 3A(1) and 3A(2), a precursor film PRC and an amorphoussilicon film ASI are formed over an entire area on an insulatingsubstrate INS by a chemical vapor deposition method (a CVD method). Theprecursor film PRC is not necessarily formed, depending upon the kind ofthe insulating film. In this example, the precursor film PRC isprovided.

The deposited amorphous silicon film ASI is converted into apolycrystalline silicon film PSI by irradiating excimer laser light EXLover an entire area of the amorphous silicon film ASI. This conversionis processing of an amorphous state into a crystalline state. Thepolycrystalline silicon film PST1 may be formed directly by using achemical vapor deposition method (a CVD method) or a sputtering method.

Next, as shown in FIGS. 3B(1) and 3B(2), formed by scanning in aspecified direction SSLD and selectively irradiating pulse-modulatedlaser light or quasi-CW laser light SSL from solid-state laser onregions of the polycrystalline silicon film PSI corresponding to theregions denoted by reference symbol SX in FIG. 1, are discontinuousconverted regions VTL comprised of even band-shaped-crystalpolycrystalline silicon films formed of crystalline grains having theirgrain sizes extended in the scanning direction of the laser light anduniform crystalline grain widths in a direction perpendicular to thescanning direction of the laser light. This conversion is processing ofenlarging polycrystalline grain sizes.

It is not always necessary that the scanning direction EXLD of theexcimer laser light EXL is coincident with the scanning direction SSLDof the pulse-modulated laser light or quasi-CW laser light SSL fromsolid-state laser.

In the following, the rectangular discontinuous converted region VTLobtained by the above-mentioned method will also be called the virtualtile for convenience' sake.

The size of the virtual tile VTL is determined by the scale of a circuitto be fabricated within the virtual tile VTL, or the size required forplural circuits to be fabricated within the virtual tile VTL.

FIGS. 4A and 4B are plan views schematically illustrating relationshipsamong a layout of a thin film transistor, the scanning directions ofsolid-state laser light and grain boundaries. The thin film transistorsof the circuits within the regions SX of FIG. 1 are fabricated by usingthe above-explained method.

In a case in which the scanning direction SSLD of the pulse-modulatedlaser light or quasi-CW laser light SSL from solid-state laser isselected to be parallel with a source-to-drain direction SDD of a thinfilm transistor TFTP as shown in FIG. 4A, since the number of timeselectrons are scattered at crystalline grain boundaries is small,electron mobility is as high as 300 cm²/V·s to 500 cm²/V·s, andvariations in threshold voltage are within ±0.2 V.

On the other hand, in a case in which the scanning direction SSLD of thepulse-modulated laser light or quasi-CW laser light SSL from solid-statelaser is selected to be perpendicular to a source-to-drain direction SDDof a thin film transistor TFTV as shown in FIG. 4B, electron mobilitybecomes as low as 100 cm²/V·s to 300 cm²/V·s, but, since the resistanceis increased, an OFF-current is small, little degradation incharacteristics occurs, the transistor exhibits high withstand-voltagetransistor characteristics.

FIG. 5 is a graph showing a comparison in transfer characteristicbetween two different layouts of transistors fabricated within thevirtual tiles VTL. Curves TFTPC and TFTVC in FIG. 5 show the transfercharacteristics of the transistors TFTP, TFTV of FIGS. 4A and 4B,respectively. Therefore, they can be used as elements for retaining,discharging and storing charges, for example, memory switches.

The above-explained thin film transistors make it possible to mount thehigh-speed circuits (mainly the pixel-driving circuits) which haveconventionally been mounted in the form of LSI chips around the imagesection (the display area) of the glass substrate, directly on the sameglass substrate. This makes it possible to reduce the cost of LSI chips,and the area of non-displaying region at the periphery of the panel (theperipheral portion of the active-matrix substrate). Further,conventionally, custom-tailoring of circuits has been performing at thedesign and fabrication of LSI chips, but the present invention makes itpossible to custom-tailor the circuits at the time of fabricating apanel intended for an active-matrix substrate.

In the following, actual exemplary layouts on actual substrates will beexplained by reference to FIGS. 6 to 9.

FIG. 6 is a plan view of a principal portion for illustrating an exampleof a layout of a pixel on an insulating substrate in an example of thepresent invention. The pixel PXL in the pixel region DSP shown in FIG. 1is comprised of a switch SW formed of thin film transistors forretaining, discharging and storing charges, a storage capacitance CST,and a pixel electrode CLQ, a source-side end of the switch SW isconnected to a drain line DL via a contact SCT, a drain-side end of theswitch SW is connected to the storage capacitance CST, and a gate of theswitch SW is connected to a gate line.

If the switch SW is in an ON state, a gray scale signal transferred tothe drain line DL is transferred to the storage capacitance CST and thepixel electrode CLQ connected to the storage capacitance CST via acontact ICT in the form of an electrical charge. Then, if the switch SWis turned in an OFF state, the electrical charge is retained, andthereafter, if the switch SW is turned into an ON state again, theelectrical charge is discharged into the drain line DL, and then thepixel is reset.

The switch SW has a double-gate structure comprised of two thin filmtransistors SWTR1 and SWTR2 so as to improve withstand voltages.Although, in this example, the switch SW is illustrated as having thedouble-gate structure, the improvement on withstand voltages can also berealized by adopting a switch comprised of one or two thin filmtransistors having an LDD (Lightly Doped Drain) structure to beexplained subsequently. The elements constituting the pixel PXL arefabricated from the above-explained polycrystalline silicon film PSIconverted from amorphous silicon by using excimer laser. Consequently,no anisotropy is present in the polycrystalline silicon film, and as aresult, regardless of what the layout of a thin film transistor is,there is little difference in characteristics of the thin filmtransistor.

Therefore, in this example, for the purpose of reducing an area of thelayout and improving the aperture ratio of the pixel, it is desirablethat the source-to-drain directions of the two thin film transistorsSWTR1 and SWTR2 are selected to be perpendicular to each other in thelayout.

As in the case of the pixels PXL, constituent elements of the samplingswitches SSW, the drain-side level shifter DLS, the gate-side levelshifter GLS, and the buffers BF are fabricated from the polycrystallinesilicon film PSI explained in connection with FIGS. 3A(1) to 3B(2).Therefore, it is desirable that the source-to-drain directions of someof thin film transistors constituting the above-mentioned circuits areselected to be perpendicular to each other in the layout.

FIG. 7 is a plan view of a principal portion for schematicallyillustrating an example of a layout of one stage constituting thedrain-side shift register DSR on the substrate. FIG. 8 is a circuitdiagram of a logic circuit corresponding to the one stage constitutingthe drain-side shift register DSR. FIG. 9 is a timing chart forexplaining operation of the shift register DSR shown in FIG. 7.

Usually the shift register constituting a driving circuit of an imagedisplay device is comprised of a number M of stages. Here consider anNth one of the M stages for convenience' sake. A signal SOUTN-1outputted from the (N−1)th stage of the shift register is outputted as asignal SOUTN via two clocked inverters. Clock signals for the (N+1)thstage are changed to those for the Nth stage, and by controlling theclock signals CLK1 and CLK2 as indicated in the timing chart of FIG. 9,the rising time of signals at the respective stages can be shifted. As aresult, timing at which display data are transferred to a gate of a thinfilm transistor or a drain line can be shifted by a time DELHsuccessively for each of the pixels.

The circuit of the shift register DSR can be realized in various ways,and when a circuit configuration shown in FIG. 8 is adopted, the numberof elements constituting the circuit can be reduced, but the rising andfalling of the signals need to be steep.

The constituent elements of the drain-side shift register DSR arefabricated within the virtual tile VTL shown in FIGS. 3B(1) and 3B(2).This circuit is required to perform high-speed operation, and thereforeit is desirable that the source-to-drain direction of all the thin filmtransistors is selected to be parallel with the scanning direction SSLDof the pulse-modulated laser light or quasi-CW laser light SSL fromsolid-state laser in the layout.

Further, as in the case of the drain-side shift register DSR, theinterface IF, the clock signal generator circuits CLG, the gate-sideshift register GSR and the digital-to-analog converter circuit DSC arefabricated within the virtual tiles VTL. Therefore, it is desirable thatthe source-to-drain directions of all the thin film transistorsconstituting these circuits are selected to be parallel with thescanning direction of the pulse-modulated laser light or quasi-CW laserlight SSL from solid-state laser in the layout.

Further, it is an object of the present invention to provide an optimumlayout for an arbitrary circuit specification, and therefore the presentinvention is not limited to the driving system and circuit arrangementas described in the above-described example, but are applicable to otherdriving systems and circuit arrangements. For example, consider adigital-to-analog converter circuit provided with memories. As explainedpreviously, for the elements for retaining, discharging and storingelectric charges, it is desirable to adopt the thin film transistorshaving a layout in which the thin film transistors are arrangedperpendicularly to the scanning direction of the pulse-modulated laserlight or quasi-CW laser light SSL from solid-state laser. That is tosay, in this case, it is desirable that the thin film transistorsserving as the switching elements for memories are arranged to beperpendicular to the scanning direction SSLD of the pulse-modulatedlaser light or quasi-CW laser light SSL, and that the other constituentthin film transistors of the digital-to-analog converter circuit arearranged to be parallel with the scanning direction SSLD. Similarly,among the high-speed circuits fabricated within the virtual tiles VTL, acircuit including a thin film transistor for retaining, discharging anddischarging charges, such as a memory, is configured such that only thisthin film transistor is arranged perpendicularly to the remainder of thetransistors constituting the circuit.

The optimum structure is selected for a thin film transistor accordingto its circuit specification. For example, it is known that transistorperformances are improved and their variations are reduced by thinning agate insulating film of a thin film transistor or using a insulatingfilm of a high dielectric constant as the gate insulating film. FIG. 10shows a difference in characteristics due to a difference in filmthickness between silicon oxide films used as gate insulating films byplotting drain currents against gate voltages. As is apparent from FIG.10, curve TFT50 for a silicon oxide film thickness of 50 nm exhibitsbetter rising characteristics and larger currents compared with curveTFT100 for a silicon oxide film thickness of 100 nm. Therefore, by wayof example, the circuit performance can be improved further by usingthin film transistors of a thin gate insulating film for low-voltagehigh-speed circuits such as the shift registers, the digital-to-analogconverter circuit and the interface, and by using thin film transistorsof a thick gate insulating film for the remainder of the circuits.

FIGS. 11A and 11B are schematic cross-sectional views of thin filmtransistor structures in one case in which impurity-injected layers oflow concentration are present in boundaries between a channel region anda source region and between the channel region and a drain region, andin another case in which a gate is disposed over impurity-injectedlayers of low concentration, respectively. FIG. 11A illustrates the casein which the impurity-injected layers of low concentration are presentin boundary regions between the channel region and the source region andbetween the channel region and the drain region, and FIG. 11Billustrates the case in which the gate is disposed over theimpurity-injected layers of low concentration.

In the case where the thin film transistor TFT is adopted which employsan LDD (Lightly Doped Drain) structure having the impurity-injectedlayers LDDR of low concentration present in boundary regions between thechannel region CHR and the source and drain regions SDR as shown in FIG.11A, although its performance deteriorates, an OFF-current can besuppressed which produces a parasitic transistor causing a problem in anordinary transistor, and its reliability is also improved. Consequently,it is desirable to adopt the LDD structure in a circuit requiring a lowleakage current, such as a pixel circuit; a circuit requiring a highwithstand voltage and high reliability, such as a level shifter and abuffer; or a circuit required to avoid variations in a gray scalevoltage caused by an increase in the Early voltage due to a parasiticbipolar operation, such as a gray-scale voltage generator circuit.

In the case where a GOLD (Gate Overlapped LDD) structure is adoptedwhich has regions GOLD where the gate is formed over the LDD regionsLDDR as shown in FIG. 11B, its performance is improved over the LDDstructure, and its reliability is improved, and consequently, thecircuit performance can be improved further.

In consideration of the above, the following explains an example of anactive-matrix substrate constituting an image display device. FIG. 12 isa plan view schematically illustrating a concept of anotherconfiguration example of an active-matrix substrate in accordance withthe present invention. The arrangement of the respective circuits inFIG. 12 is based on that shown in FIG. 1.

In a region RGN1 where pixels and circuits requiring high withstandvoltages are mounted, some of the thin film transistors TFT1constituting those circuits have their source-to-drain directionsoriented in parallel with the scanning direction SSLD of the solid-statelaser, and others have their source-to-drain directions orientedperpendicularly to the scanning direction SSLD. To be concrete, it isdesirable that the thin film transistors TFT1 employ the above-explainedLDD or GOLD structures.

In a region RGN2 where high-performance circuits are mounted, all thethin film transistors TFT2 constituting the circuits have theirsource-to-drain directions oriented in parallel with the scanningdirection SSLD of the solid-state laser. To be concrete, the thin filmtransistors TFT2 may employ the LDD or GOLD structures, and if they areoperated at low voltages, they do not need high withstand voltage, andtherefore it is desirable that they employ a simple complementary MOS(Metal Oxide Semiconductor) structure. Further, it is desirable that thegate insulating film of the thin film transistors TFT2 is selected to besmaller in thickness than that of the thin film transistors TFT1, orthat the gate insulating film of the thin film transistors TFT2 isfabricated from a material having a high permittivity.

In a region RGN3 where circuits for generating gray scale signals aremounted, all the thin film transistors TFT2 constituting the circuitshave their source-to-drain directions oriented in parallel with thescanning direction SSLD of the solid-state laser. To be concrete, it isdesirable that the thin film transistors TFT2 employ the LDD or GOLDstructures to suppress the parasitic bipolar operation.

Fabrication of the thin film transistors in accordance with the presentinvention may be carried out by repeating well-known oxidation,film-formation and photolithography process steps with theabove-explained layouts considered in circuit design. The only processspecial to the present invention is determination of positions of thevirtual tiles VTL. The following explains a method of determining thepositions of the virtual tiles VTL.

FIGS. 13A(1) to 13B(2) are illustrations for explaining the method ofpositioning regions to be irradiated by solid-state laser light fordetermining the positions of the virtual tiles VTL. FIG. 13A(1) is aperspective view illustrating formation of a positioning mark on asubstrate, FIG. 13A(2) is a plan view of the substrate of FIG. 13A(1),as viewed from above, FIG. 13B(1) is a perspective view illustratingirradiation of laser light onto the substrate, and FIG. 13B(2) is a planview of the substrate of FIG. 13B(1), as viewed from above.

In FIGS. 13A(1) and 13A(2), formed on a polycrystalline silicon film PSIby using a photolithographic method, a dry etching method, or laser, isa positioning mark MARK which serves as a target for determining aposition to be irradiated by pulse-modulated laser light or quasi-CWlaser light. Any of the above methods may be employed for formation ofthe positioning mark MARK, and when laser is used, the number of masksand the number of photolithgraphic process steps can be prevented fromincreasing.

Then, as shown in FIGS. 13B(1) and 13B(2), the pulse-modulated laserlight SXL are irradiated discontinuously onto the substrate by selectingspecified regions VTL by using the mark MK as a reference point whilethe pulse-modulated laser light SXL is scanned in a direction SSLD.After discontinuous converted regions VTL comprised ofband-shaped-crystal polycrystalline silicon films are fabricated byscanning and irradiating the pulse-modulated laser light SXL onto theregions VTL, well-known conventional fabrication steps for thin filmtransistors may be adopted.

FIG. 14 is an exploded perspective view for schematically explaining aconfiguration example in which an image display device of the presentinvention is applied to a liquid crystal display device. Fabricated on aglass substrate SUB1 constituting an active-matrix substrate are aplurality of pixel electrodes PXL arranged in a matrix fashion, adrain-side circuit DSR and a gate-side driving circuit GSR for inputtingdisplay signals into the pixel electrodes, and a group of circuits CIRnecessary for image display. Then an alignment layer LO is coated on theglass substrate SUB1 by using a printing method, and then aliquid-crystal aligning capability is imparted to the alignment layer LOas by using a rubbing method.

On the other hand, a color filter CF and a counter electrode ITO arefabricated on a counter substrate SUB2, and thereafter an alignment filmLO is coated on the counter substrate SUB2, and then the liquid-crystalaligning capability is imparted to the alignment layer LO in the sameway as above. The counter substrate SUB2 is attached to the glasssubstrate SUB1 with a spacing therebetween, then a liquid crystalmaterial LIQ is filled between the opposing alignment layers LO byvacuum injection, and then the counter substrate SUB2 and the glasssubstrate SUB2 are sealed along their peripheries by using a sealingmaterial SEA. In this case, spacers SPC interposed between the countersubstrate SUB2 and the glass substrate SUB1 establish the spacingbetween the two substrates. In most cases, plastic beads or glass beadsare used as the spacers. Instead, columnar spacers can be used which arefabricated on the counter substrate SUB2 or the glass substrate SUB1 byusing photolithographic techniques.

Thereafter, a polarizer DEF is attached on a surface of each of theglass substrate SUB1 and the counter substrate SUB2. Then a liquidcrystal display device is completed by attaching a backlight BKL behindthe glass substrate SUB1.

Incidentally, although FIG. 14 illustrates the example in which a colorfilter is fabricated on the counter substrate SUB2, the presentinvention is equally applicable to a liquid crystal display device ofthe type having a color filter fabricated on the glass substrate SUB1which serves as an active-matrix substrate.

Further, an organic EL display device can also be fabricated by usingthe active-matrix substrate explained in connection with FIGS. 1 to13(b-2). FIG. 15 is an exploded perspective view illustrating aconfiguration example in which an image display device of the presentinvention is applied to an organic EL display device. FIG. 16 is a planview of the organic EL display device obtained by assembling theconstituent elements shown in FIG. 15.

An organic EL element is fabricated on each of the pixel electrodes onthe active-matrix substrate SUB explained in the above-describedexample. The organic EL element is formed of a stack of successivelyevaporated layers comprising a hole transport layer, a light-emittinglayer, an electron transport layer, and a cathode metal layer from asurface of the pixel electrode in the order named. Each of the organicEL elements is provided with a pixel circuit formed of thin filmtransistor circuits (not shown). A driving circuit section DDR and ascan-driving circuit section GDR are fabricated outside of a pixelregion PAR. The driving circuit section DDR and the scan-driving circuitsection GDR are supplied with display signals and scan signals from anexternal signal source via a flexible printed circuit board PLB. Thedriving circuit section DDR and the scan-driving circuit section GDR areformed of the above-explained thin film transistors. An integratedcircuit constituting a display control device CTL is mounted on theflexible printed circuit board PLB.

The active-matrix substrate SUB provided with the above-mentioned stackof the evaporated layers is sealed by using a sealing substrate SUBX ora sealing can with a sealing material disposed around the pixel regionPAR. An organic EL display device is completed by combining theactive-matrix substrate SUB with a shield frame SHD which serves as anupper case and a lower case CAS, in an integral structure.

In active-matrix driving of the organic EL display device, since theorganic EL element is of the current-driven light emission type, theadoption of high-performance pixel circuits is essential for providing ahigh-quality image, and it is desirable to employ pixel circuits formedof CMOS type thin film transistors. Further, the thin film transistorcircuits fabricated in the driving circuit regions are indispensable forrealizing high-speed operation and a high-definition display. Theactive-matrix substrate SUB in this example has high performancescapable of meeting such requirements. The organic EL display deviceemploying the active-matrix substrate of this example is one of thedisplay devices capable of making the most of the advantages of thisexample.

The present invention is not limited to the display devices employingthe above-described active-matrix substrates, the configurations definedin claims of this specification, or the configurations explained in theembodiments of this specification, but various changes and modificationsmay be made without departing from the sprit of the present invention,and for example, the present invention can be applied to various kindsof semiconductor devices.

FIGS. 17 to 20 illustrate exemplary display devices to which the presentinvention is applied.

FIG. 17 is an external view of an example in which an image displaydevice of the present invention is incorporated into a display sectionof a personal computer or a TV receiver, and in which a liquid crystaldisplay device LIQMON of the present invention is employed in a displaysection MON of the personal computer or the TV receiver.

FIG. 18 is an external view of an example in which an image displaydevice of the present invention is incorporated into a display sectionof a mobile phone, and in which a liquid crystal display device LIQMONof the present invention is employed in a display section MOB of themobile phone.

FIG. 19 is an external view of an example in which an image displaydevice of the present invention is incorporated into a display sectionof a portable digital terminal, and in which a display device LIQMON ofthe present invention is employed in a display section of the portabledigital terminal PDA.

FIG. 20 is an external view of an example in which an image displaydevice of the present invention is incorporated into a display sectionof a video camera, and in which a display device LIQMON of the presentinvention is employed in a viewfinder section of the video camera CAM.

In addition to the above, an image display device of the presentinvention can be employed in an image display section of a digital stillcamera, a projector, a vehicle navigation system, or the like.

As described above, the present invention provides a high-quality-imagedisplay device provided with a matrix-array pixel section and apixel-driving circuit section employing high-performance thin filmtransistor circuits operating with high-speed current mobility fordriving the pixel sections, obtained by optimizing the layouts ofvarious kinds of circuit sections on an insulating substrateconstituting an active-matrix substrate.

1. An image display device provided with an active-matrix substratecomprising: an insulating substrate; and a plurality of circuit regionsfabricated on said insulating substrate and including at least a pixelsection and a pixel-driving circuit section, each of said pixel sectionand said pixel-driving circuit section having a polycrystalline siliconsemiconductor film, wherein said plurality of circuit regions includethin film transistors having a lightly doped drain (LDD) structure;wherein at least one of said plurality of circuit regions has a firsttype of a thin film transistor and a second type of a thin filmtransistor, and an angular orientation of a direction of a currentflowing through a channel of said first type of a thin film transistoris formed to be non-parallel with an angular orientation of a directionof a current flowing through a channel of said second type of a thinfilm transistor; wherein said plurality of circuit regions includes atleast one pair of a first-type circuit region constituting a firstcircuit and a second-type circuit region constituting a second circuit,all thin film transistors in said first-type circuit region flowcurrents through channels thereof in one angular orientation, andangular orientations of currents flowing through channels of thin filmtransistors in said second-type circuit 10 region are plural in number;and wherein, in said first-type circuit region, a peak-to-valley heightdifference of a surface of said channel, a source region and a drainregion of said thin film transistors is equal to or smaller than 5 nm,and crystalline grains of said polycrystalline silicon semiconductorfilm are of a rectangular shape of 0.3 μm to 2 μm in width and 4 μm ormore in length, and in said second-type circuit region, an averagecrystalline grain diameter is 1 μm or smaller and a peak-to-valleyheight difference of a surface is equal to or greater than 20 nm, insaid channel, a source region and a drain region of said thin filmtransistors.
 2. An image display device according to claim 1, wherein insaid polycrystalline silicon films in a channel, a source region and adrain region of said thin film transistors constituting said pixelsection, an average crystalline grain diameter is 1 μm or smaller, and apeak-to-valley height difference of a surface is equal to or greaterthan 20 nm; and wherein in at least one of said plurality of circuitregions excluding said pixel section, crystalline grains of saidpolycrystalline silicon films are of a rectangular shape of 0.3 μm to 2μm in width and 4 μm or more in length in a channel, a source region anda drain region of said thin film transistors, and a peak-to-valleyheight difference of a surface of said channel, said source region andsaid drain region of said thin film transistors is equal to or smallerthan 5 nm.
 3. An image display device according to claim 1, wherein saidthin film transistors have plural kinds of gate insulating materials andplural kinds of thickness in ones of said plurality of circuit regionsexcluding said one of said plurality of circuit regions constitutingsaid pixel section.
 4. An image display device according to claim 1,wherein a level shifter, a sampling switch circuit and a buffer circuit25 constituting a pixel-driving circuit are fabricated in ones of saidplurality of circuit regions excluding said one of said plurality ofcircuit regions constituting said pixel section, said channel, saidsource region and said drain region of said thin film transistorsconstituting said pixel-driving circuit are formed of polycrystallinesilicon films having an average crystalline grain diameter of 1 μm orsmaller and a peak-to-valley height difference of a surface equal to orgreater than 20 nm, and said channel, said source region and said drainregion of said thin film transistors constituting at least one of saidcircuits excluding said level shifter and said sampling switch circuitare formed of polycrystalline silicon films having crystalline grains ofa rectangular shape of 0.3 μm to 2 μm in width and 4 μm or more inlength and a peak-to-valley height difference of a surface equal to orsmaller than 5 nm.
 5. An image display device provided with anactive-matrix substrate comprising: an insulating substrate; and aplurality of circuit regions fabricated on said insulating substrate andincluding at least a pixel section and a pixel-driving circuit sectioneach of said pixel section and said pixel-driving circuit section havinga polycrystalline silicon semiconductor film, wherein said plurality ofcircuit regions include thin film transistors having a lightly dopeddrain (LDD) structure; wherein at least one of said plurality of circuitregions has a first type of a thin film transistor and a second type ofa thin film transistors and an angular orientation of a direction of acurrent flowing through a channel of said first type of a thin filmtransistor is formed to be non-parallel with an angular orientation of adirection of a current flowing through a channel of said second type ofa thin film transistor; wherein said plurality of circuit regionsincludes at least one pair of a first-type circuit region constituting afirst circuit and a second-type circuit region constituting a secondcircuit, all thin film transistors in said first-type circuit 15 regionflow currents through channels thereof in one angular orientation, andangular orientations of currents flowing through channels of thin filmtransistors in said second type circuit region are plural in number,wherein, in said first-type circuit region, a peak-to-valley heightdifference of a surface of said channel, a source region and a drainregion of said thin film transistors is equal to or smaller than 5 nm,and crystalline grains of said polycrystalline silicon semiconductorfilm are of a rectangular shape of 0.3 μm to 2 μm in width and 4 μm ormore in length, and in said second-type circuit region, an averagecrystalline grain diameter is 1 μm or smaller and a peak-to-valleyheight difference of a surface is equal to or greater than 20 nm, insaid channel, a source region and a drain region of said thin filmtransistors; and wherein a thickness of gate insulating films of saidthin film transistors in said first-type circuit region is smaller thana thickness of gate insulating films of said thin film transistors insaid second-type circuit region.
 6. An image display device according toclaim 5, wherein in said polycrystalline silicon films in a channel, asource region and a drain region of said thin film transistorsconstituting said pixel section, an average crystalline grain diameteris 1 μm or smaller, and a peak-to-valley height difference of a surfaceis equal to or greater than 20 nm; and wherein in at least one of saidplurality of circuit 20 regions excluding said pixel section,crystalline grains of said polycrystalline silicon films are of arectangular shape of 0.3 μm to 2 μm in width and 4 μm or more in lengthin a channel, a source region and a drain region of said thin filmtransistors, and a peak-to-valley height difference of a surface of saidchannel, said source region and said drain region of said thin filmtransistors is equal to or smaller than 5 nm.
 7. An image display deviceaccording to claim 5, wherein said thin film transistors have pluralkinds of gate insulating materials and plural kinds of thickness in onesof said plurality of circuit regions excluding said one of saidplurality of circuit regions constituting said pixel section.
 8. Animage display device according to claim 5, wherein a level shifter, asampling switch circuit and a buffer circuit constituting apixel-driving circuit are fabricated in ones of said plurality ofcircuit regions excluding said one of said plurality of circuit regionsconstituting said pixel section, said channel, said source region andsaid drain region of said thin film transistors constituting saidpixel-driving circuit are formed of polycrystalline silicon films havingan average crystalline grain diameter of 1 μm or smaller and apeak-to-valley height difference of a surface equal to or 20 greaterthan 20 nm, and said channel, said source region and said drain regionof said thin film transistors constituting at least one of said circuitsexcluding said level shifter and said sampling switch circuit are formedof polycrystalline silicon films having crystalline grains of arectangular shape of 0.3 μm to 2 μm in width and 4 μm or more in lengthand a peak-to-valley height difference of a surface equal to or smallerthan 5 nm.